1. Field of the Invention
This invention relates to optical systems for constituting image display apparatuses, particularly, relates to illumination optical systems and projection optical systems suitable for constructing an image display apparatus in which plural images having respective colors are formed on a single and common plane at every.
2. Description of the Related Arts
As an optical system in which plural images are formed on a single and common plane with respective colors, there is disclosed a tri-color separation optical system in Japanese Patent Laid-open Publication, 1-68190/1989, filed by the present Applicant.
This optical system is constructed as a tri-color separation system for an image sensing device as shown in FIG. 1. Description is simply given of a principle thereof.
FIG. 1 is a schematic view showing a tri-color separation optical system in the prior art. Referring to FIG. 1, an object "O"is focused on an image plane "F"by an image sensing lens "PL". A dichroic prism Dp is disposed in an optical path. In the dichroic prism Dp, a reference character BDM denotes a B-dichroic mirror surface for reflecting a blue color and transmitting green and red colors, and RDM an R-dichroic mirror surface for reflecting a red color and transmitting green and blue colors. Reference characters Mr and Mb denote total reflection mirrors, Pr an optical path compensating prism for the red color and Pb an optical path compensating prism for the blue color. The optical path compensating prisms Pr, Pb are respectively used for both a red color optical path and a blue color optical path to cause their optical path lengths from the image sensing lens PL to the image plane F to be equal to that of a green color which passes straight the dichroic prism Dp. Thus, the red, blue and green colors images are respectively focused on the common plane by the functions of the optical compensating prisms Pr, Pb.
A white light beam impinging on the image sensing lens PL from the object 0 further impinges on the dichroic prism Dp.
Among a white light beam impinging on the dichroic prism Dp, a green color component beam passes through the R-dicrhoic and B-dicrhoic mirror surfaces RDM, BDM, and is focused on the image plane F as a displayed image Ig.
Among the white light beam impinging on the dichroic prism Dp, a blue color component beam is reflected by the B-dicrhoic mirror surface BDM, and impinges on the optical compensating prism Pb. Then, the blue color component beam impinging on the B-optical compensating prism Pb is reflected by the total reflection mirror Mb, and is focused on the image plane F as a displayed image Ib.
Among the white light beam impinging on the dichroic prism Dp, a red color component beam is reflected by the R-dicrhoic mirror surface RDM, and impinges on the optical compensating prism Pr. Then, the red color component beam impinging on the R-optical compensating prism Pr is reflected by the total reflection mirror Mr, and is focused on the image plane F as a displayed image Ir.
As a result, the image of the objective O is obtained on the image plane F as three displayed images Ib, Ig, Ir by being separated into three colors, blue, green and red.
In this case, the description is given of the tri-color separation optical system for the image sensing device.
However, this principle is applicable to the tri-color separation optical system for the illumination optical system by substituting the object O with a light source. Thus, the three light source images are obtained on a common image plane with respective colors.
This principle is also applicable to a tri-color composition optical system by reversing these optical paths.
As an exemplary display device employing the optical system shown in FIG. 1, there is a color image display device disclosed in Japanese Patent Laid-open Publication, 3-288124/1991, filed by the present Applicant.
This color image display device is shown in FIG. 2.
FIG. 2 is a perspective view showing a color image display apparatus in the prior art.
The description is simply given of the principle of the color image display apparatus.
Referring to FIG. 2, when a light beam from a light source LAMP impinges on a polarizing beam splitter PBS, a light beam having a component mainly oscillating in a direction perpendicular to a plane of incidence defined by a normal line of a boundary surface and an incident ray is reflected by a boundary surface thereof in a direction of a tri-color separation optical system SCA having the same construction as one shown in FIG. 1. The light beam is separated into the three color RGB light beams in the tri-color separation optical system SCA in the same manner as mentioned in the foregoing, and each of them impinges on a reflection type spatial light modulation element SLMr. The three color RGB light beams are reflected by the reflection type spatial light modulation element (referred to as display element) SLMr, and back-track the optical path to the tri-color separation optical system SCA, and impinge on the polarizing beam splitter PBS. The polarizing beam splitter PBS causes the light beam having a component oscillating in a direction parallel to the plane of incidence to pass through the boundary surface thereof. Thus, these light beams are projected by a projection lens PL by being magnified.
However, when the prisms shown in FIG. 1 are applied to the tri-color separation optical system SCA shown in FIG. 2, effective light beams among the light beams incident on the optical system from the light source correspond to ones impinging on areas EA of the display element SLMr. In other words, effective apertures of the display element SLMr becomes smaller than those of the optical system. Thus, the availability factor of the, display element SLMr is decreased, resulting in a cost problem of the display element.
When three individual pieces of the display elements SLMr are disposed on a single plane, it requires the prisms having larger apertures than effective areas of the display element. This poses problems of a large size of the optical system and of a decrease of the availability factor of the light because of a long optical path.
In the color composition optical system, the same problem occurs. In order to effectively utilize the light beam reflected by the display element, it requires an objective lens having a larger aperture. Thus, the prism needs an aperture as large as that of the objective lens.
Further, when the tri-color separation optical system shown in FIG. 2 is constructed by using the prisms made of optical glass, a birefringence occurs due to the optical glass. This poses a problem of a decrease of a contrast ratio.
These problems occur not only in the case of the reflection type display element but also in the case of the transmission type display element.